The invention relates to a method of operating an internal combustion engine selectively either on all cylinders or with only certain cylinders in operation.
A number of internal combustion engines are known which can be operated either on all engine cylinders (full engine) or with certain cylinders cut off. Cylinder cut-off is used in particular in large capacity internal combustion engines with more than six cylinders in order to improve the engine operating efficiency under partial load operating conditions. By means of the cylinder cut-off, some of the cylinders, namely the group of cut-off cylinders, are excluded from performing work, so that the remaining group of operative cylinders that continue to fire even in the event of cylinder cut-off are operated at increased specific load with correspondingly better specific fuel economy, whilst maintaining the required engine power output at improved operating efficiency. This advantage is offset by the disadvantage that, over the life of the internal combustion engine, this results in unequal operating times for the group of cut-off cylinders and the group of operating cylinders, which continue to fire even in the event of cylinder cut-off. Such operation results in correspondingly unequal wear of the components involved in the charge cycle and in performing work. The group of inoperative cylinders cools during the periods in which they are cut off, resulting in increased friction, a tendency to form deposits and, depending on the design of the exhaust line, difficulties in maintaining a uniformly high exhaust gas temperature level, which is necessary for exhaust emission control by means of catalytic converters, especially when switching again back to full engine operation.
In internal combustion engines with cylinders divided into a group of cylinders that can be made inoperative and a group of cylinders which are always operating as disclosed for example by DE 44 21 257 A1, attempts are made to counter the latter difficulties by also maintaining the charge cycle for the cut-off cylinders during cylinder cut-off operation. In this case only the fuel supply is cut off, and the overall exhaust system is designed so that the air flowing through the inoperative group of cylinders is heated by the exhaust gases of the group of cylinders which are operating.
It is the object of the present invention to provide a method for the operation of an internal combustion engine selectively either with all cylinders operating or with certain cylinders made inoperative in such a way that unequal loads and unequal wear of the individual groups of cylinders as well as greater fluctuations of the exhaust gas temperatures resulting from the cylinder cut-off are avoided.
In a method for the operation of an internal combustion engine either on all cylinders (as full engine) or with only some cylinders in operation (cylinder cut-off), the cylinders are divided, in the firing order, into two groups of alternating cylinders which are selectively operative with an angular ignition spacing twice that of the cylinders of the engine when operating as full engine, the two groups of cylinders being activated alternately during engine operation with cylinder cut-off.
If no preferences are set, the alternating cut-off of the groups of cylinders leads to essentially equal running times for the groups of cylinders. When the engine is operating on all cylinders, the cylinders of both groups are operative. For partial cylinder operation, the running times for the cut-off of the groups of cylinders can be selected in a way that the fluctuations in exhaust emission temperatures by and large remain in a range as needed for proper operation of the exhaust emission control devices, especially the catalytic converters. This is facilitated, according to the invention, in that the operating cycle elements (valves and ignition) for the group of the cylinders made inoperative at any given time are shut down, so that the charge cycle for the inoperative group of cylinders is interrupted. In such a solution moreover prevents pumping losses that adversely affect the efficiency of the internal combustion engine.
The method according to the invention does not affect the capability to switch to full engine operation at any time according to demand, that is, as a function of the required power output. But even under the given conditions differences may occur in the operating times of the groups of cylinders. Given a continuing existence of the operating conditions permitting cylinder cut-off operation, the invention therefore limits the duration of the operating period for a group of cylinders operating at any one time. The limit is determined either as a function of time, or as a function of certain operating parameters of the internal combustion engine including the associated exhaust system, or by a combination of time and operating parameters. It is thus possible, within the scope of the invention, to influence and, in particular, to provide a maximum time for the operating period of a group of cylinders which operate during cylinder cut-off, subject to driving and/or other operating parameters.
During an extended operating condition permitting cylinder cut-off operation, the switch from one to the other group of cylinders can, according to the invention, be effected without interruption. However, an especially effective way resides in the intercalation of an operating period in which the internal combustion engine is operated as full engine. This has proved effective with a view to obtaining the most uniform output conditions possible for all cylinders during the respective cut-off mode of operation when a change between the groups of cylinders is initiated. Moreover, this also improves the switchover comfort.
The full engine operating period proposed as intermediate phase when changing from the operation of one group of cylinders to the operation of another group of cylinders can be kept very brief. In particular, it may be significantly shorter than the pre-set the maximum period of operation for a group of cylinders during a continuing existence of operating conditions permitting cylinder cut-off operation.
The method according to the invention has been found effective, especially in connection with internal combustion engines having more than six cylinders. Large capacity internal combustion engines are especially suitable for application of the method.
Thus it proves effective, for example, to apply the invention to an 8-cylinder engine, especially an 8-cylinder V-engine. The cylinders of a V8 engine are numbered, looking at the engine from the front, 1, 2, 3, 4 for the left bank of cylinders an 5, 6, 7, 8 for the right bank of cylinders. In that case, two cylinder groups can be formed, the cylinders of each of which have the same angular ignition spacing from one another. Divided into two groups in the firing order, the angular ignition spacing for the operating mode with cylinder cut-off, however, is twice the angular ignition spacing during full-engine operation. Thus for an 8-cylinder engine with the firing order 1-5-4-2-6-3-7-8, for example, this results in two groups of cylinders (1+4+6+7) for one group of cylinders and (2+3+8+5) for the other group. The cylinders are selected in the firing order. The cylinder cut-offs of the two groups are initiated alternately.
The method according to the invention can be implemented virtually without any additional cost especially in connection with internal combustion engines having individually actuated charge cycle control elements. It is possible, for example and advantageous to use valves with solenoid actuation as charge cycle control elements. With this type of valve actuation, it is possible at any time to keep the valves of the cut-off cylinders closed for the duration of the cut-off operating mode, so that a charge cycle is omitted, the associated losses due to the charge cycle work are avoided and shorter operating times with corresponding reduced wear are achieved for the charge cycle control elements. Upon switching from the cut-off operating mode to all cylinder operation, actuation of the valves can be resumed according to the normal valve timing.